(a) Field of the Invention
The present invention relates to an optical wavelength conversion apparatus and method. More specifically, the present invention relates to an apparatus and method for converting the wavelength of an optical signal using the injection locking of a Fabry-Perot laser diode.
(b) Description of the Related Art
An electrical wavelength conversion method, one of optical wavelength conversion techniques, converts an optical signal to an electric signal and transmits the electric signal to an optical transmitter with a different wavelength. This method has problems that there is a limit in the operating speed of an electronic device, and O/E/O conversion increases the cost.
Accordingly, a variety of all-optical wavelength conversion techniques that optically convert the wavelengths of optical signals without converting them to electric signals have been being studied. One of the techniques is cross gain modulation using a gain saturation characteristic of a semiconductor optical amplifier (referred to as SOA hereinafter). This technique provides a simple configuration and relatively stable operation characteristics. However, it requires a high input power, deteriorates the extinction ratio of an output optical signal, and increases the cost due to the expensive SOA.
Another all-optical wavelength conversion method is a cross phase modulation technique using a phase modulation difference according to a gain difference of SOAs. This technique provides a high extinction ratio because an output signal is digitally modulated on the basis of a variation of 180° of phase. However, the cross phase modulation technique requires an interferometer system to obtain cross phase modulation and needs accurate design and control for the purpose of attaining a correct phase difference.
Another all-optical wavelength conversion technique uses four-wave mixing that is a non-linear characteristic in the SOA. This is independent on a bit rate or format of an input optical signal, and enables multi-channel wavelength conversion because it uses the four-wave mixing. However, this technique uses the non-linear effect so that conversion efficiency is low. To solve this problem, the technique requires very high input optical power of more than several mW.
Recently, optical wavelength conversion that injection-locks probe and pump optical signals in an inexpensive Fabry-Perot laser diode to convert the wavelength of optical signals has been proposed. In this technique, a continuous wave optical signal (probe optical signal) is injection-locked in a specific mode of the Fabry-Perot laser diode, and an input optical signal (pump optical signal) is input to injection-lock the Fabry-Perot laser diode in another mode. When the pump optical signal is 1, the refractive index of the Fabry-Perot laser diode is changed so that modes in the Fabry-Perot laser diode are red shifted. Here, the injection locking of the probe optical signal is cancelled and its gain is lost. On the other hand, when the pump optical signal is 0, the modes are returned to their original positions so that the probe optical signal is injection-locked again and obtains a gain. In this manner, data of the pump optical signal are transferred in an inverted form to the probe optical signal. Accordingly, this method can be considered as a cross gain modulation technique that modulates the gain the probe optical signal obtains by the injection locking through an input optical signal (pump optical signal).
However, the injection locking can occurs only in the modes that obtain gains in a gain spectrum of the Fabry-Perot laser diode. Thus, a range of wavelength conversion is restricted by the gain spectrum of the Fabry-Perot laser diode.
Furthermore, in the case of non-inverting wavelength conversion, the probe optical signal and pump optical signal should be simultaneously injection-locked in two specific modes of the Fabry-Perot laser diode when the pump optical signal is 1, and then the injection locking is simultaneously cancelled when the pump optical signal is 0. However, the non-inverting wavelength conversion is very difficult to carry out because the probe optical signal has a characteristic that the injection locking of the probe signal is not cancelled even if the pump optical signal becomes 0 and the injection locking of the pump optical signal is cancelled.